Previously, hitches for transporting highway trailers on transportation vehicles, particularly on railway cars have utilized rubber-in-compression cushioning in the diagnoal strut. This is described, for example, in U.S. Pat. No. 3,145,006. Another construction illustrated in U.S. Pat. Nos. 3,246,866 and 3,512,739 utilizes hydraulic cushioning in the diagonal strut.
The construction according to the U.S. Pat. No. 3,145,006 requires too large a cushioning unit and particularly in the collapsed position the cushioning unit takes up too much room. Presently in the collapsed position the hitch may not extend vertically above the deck more than 6 inches, to avoid interference with containers to be mounted on pedestals whose mounting surfaces are 6 inches above the deck.
U.S. Pat. No. 3,493,207 discloses a tractor operated hitch which is cushioned at the base of the diagonal strut with a rubber-in-shear cushioning unit. However, this unit requires a separate housing, and does not utilize the design envelope of the diagonal strut and is thus expensive. Furthermore, some trailers have a transverse rod connecting the dolly wheels. The rubber-in-shear unit interferes with the transverse rod. Thus hitches using rubber-in-shear cushioning at the base of the strut require special operator handling. The trailers having this rod must be raised to clear the cushioning unit according to the U.S. Pat. No. 3,493,207 construction. Thus it is not considered desirable to mount the cushioning unit at the base of the diagonal strut.
The hitch of the present invention is designed to take a 10 mph impact in service. It is believed that this is the largest impact speed encountered in most railway switching yards and this impact speed occurs relatively infrequently. During such a 10 mph impact, our data indicates that about 85,000 to 90,000 ft. lbs. of work are applied to the cushioning unit as a result of such impact. The present AAR maximum allowable kingpin force is 210,000 pounds. When the rubber-in-compression unit was developed, according to the U.S. Pat. No 3,145,006, there was no maximum allowable kingpin force, and the kingpin force in some instances may be as high as 250,000 pounds for a 10 mile per hour impact. Taking into account the maximum allowable kingpin force of 210,000 pounds and the 85,000 to 90,000 ft. lbs. work applied to the cushion unit and the fact that rubber-in-compression units can only absorb on the order of 10.5 ft. lbs. per cubic inch the rubber-in-compression unit would have to be in excess of 13 feet in length for a 5 inch .times. 10 inch rectangular cross section. This length is not practical. It is also preferred that the maximum hitch height of 6 inches above the deck not be achieved by requiring the formation of openings in the deck.
In accordance with the hydraullic cushion structure illustrated in the U.S. Pat. Nos. 3,246,866 and 3,512,739, inspection and maintenance costs are higher than desired. Furthermore, the original cost of the unit is considered by some to be high due to the many machined surfaces and close tolerances required in the hydraulic portion of the assembly.